1. Field of the Invention
The present invention refers to an apparatus and method for measuring the weight of a preform for optical fibres during a chemical deposition process for the formation of the preform.
2. Description of the Related Art
As is known, the most common processes for manufacturing preforms for optical fibres foresee one or more chemical deposition steps, through one or more burners, of suitable chemical substances on a cylindrical support; the chemical substances typically comprise silicon and germanium, which are deposited in the form of oxides (SiO2 and GeO2). A suction hood eliminates the discharge gases produced by the burner.
Preform manufacturing process through chemical deposition of the prior art comprise processes of the VAD type (Vapor Axial Deposition), processes of the OVD type (Outside Vapor Deposition) and processes of the MCVD type (Modified Chemical Vapor Deposition).
Typically, in processes of the VAD type, the cylindrical support is kept in vertical position through the use of a clasping member acting upon an upper end of the cylindrical support; the cylindrical support is made to rotate upon itself so as to expose all of its surface to one or more burners housed near to the lower end of the support and in a position such as to emit a flow of reactants along a direction inclined by a predetermined angle with respect to the longitudinal axis of the support. The support is then made to translate upwards so as to allow a substantially axial growth of the preform.
In processes of the OVD type, on the other hand, the cylindrical support is kept in horizontal or vertical position through the use of a pair of clasping members acting upon the opposite ends of the support; this support is made to rotate upon itself so as to expose all of its surface to one or more burners laterally mounted with respect to the support and in a position such as to emit the flow of reactants along a direction substantially perpendicular to the longitudinal axis of the support. The burner, in particular, is mounted on a support structure equipped with a motorised moving member which allows the repeated translation of the burner parallel to the cylindrical support, so as to allow a substantially radial growth of the preform along all of the sections of the support.
In processes of the MCVD type, the deposition is carried out on the inner surface of a tubular support made to rotate about its axis, again through the use of a burner translating parallel to the axis of the support itself.
For the sake of simplicity, in the rest of the present description and in the subsequent claims explicit reference shall often be made to chemical deposition processes of the OVD type.
As is known, an important instrument for controlling and analysing the performance of the chemical deposition process is given by the measurement of weight in time, and consequently of the deposition-rate, of the preform in formation during the chemical deposition process. The measurement of the weight of the preform, indeed, besides giving information on the rate and efficiency of the chemical deposition in each of its steps, also gives advantages for the subsequent spinning steps of the optical fibre from the preform. The measurement of the weight also allows the value of the ratio between cladding/core masses to be worked out and controlled; knowing such a parameter is essential for guaranteeing the optic transmissive specification of the optical fibre spun from the preform.
Different devices for measuring the weight of a preform during a chemical deposition process for the formation of the preform are known.
JP06-329432 describes a chemical deposition process of the OVD type with horizontal configuration in which the preform is made to rotate about it own axis and is made to move horizontally above a fixed burner. A pair of opposite load cells measures the weight of the preform during the chemical deposition process. In order to lessen errors in the measurement of the weight caused by thermal variations and by misalignment of the supports of the preform caused by the movement of the preform itself during the chemical deposition process, four different constructive solutions are foreseen. A first solution foresees an alignment procedure carried out manually by an operator through adjustment of the supports of the base of the entire deposition machine. Checking of the alignment is obtained through an optical system consisting of a laser positioned on one of the supports of the preform and of a detector of the position of the laser beam positioned on the other support. A second solution foresees measurement of the misalignment through two laser distance detectors positioned on the supports of the preform, which measure its position with respect to two references; the adjustment is obtained through an automatic system which acts upon the supports of the deposition machine, such supports being suitably automated. A third solution foresees the elimination of the adjustment of the alignment of the machine. In this case the system carries out a correction via software of the signal of the distance sensors which measure the alignment. The fourth solution foresees, besides the methods already quoted, a system for cooling the structure of the deposition machine to counteract the thermal variations induced by the heat produced by the burner's flame. The temperature adjustment is obtained through a circuit equipped with a circulation pump crossed by demineralised water.
The Applicant observes that all of the solutions described above foresee the use of a system for measuring the alignment of the supports of the preforms and have the aim of eliminating just the misalignment problems through manual adjustment, or software or through thermal adjustment of the entire base of the deposition machine. However, no system is foreseen for eliminating or lessening other important sources of errors in the measurement of weight which shall be better described hereafter, like for example the thermal drift of the load cells (the effect of which is generally greater with respect to that of the expansion of the materials of the machine), the weight of the systems for anchoring the preforms and of the systems for transmitting motion to the preform, the constraint reactions imparted by the hyperstatic characteristics of the system for anchoring the preform, the dynamic action of the forces which transmit motion to the preform, as well as the dynamic actions linked for example to the rotation of the preform.
JP07-215725 described a chemical deposition process of the VAD type in which the preform, supported by an anchoring system, is made to rotate about its own axis and is moved vertically through a preform-moving device. The measurement of the weight of the preform in formation is carried out through a weight sensor arranged between the mobile part of the preform anchoring system and the preform-moving system. To increase the precision in the measurement of weight, between the mobile part of the preform anchoring system and the preform it is foreseen for there to be a mechanism of the active and passive type capable of absorbing the vibrations of the movement of the preform, such vibrations being the source of errors in the measurement of weight. The Applicant observes that such a device is not capable of lessening the errors in the measurement of weight mentioned above, like for example the thermal drift of the weight sensor, the forces transmitted by the anchoring system and system for transmitting motion to the preform and the dynamic actions of the systems for transmitting motion to the preform.
Further devices for measuring the weight of a preform during a chemical deposition process for the formation of the preform are described in JP06-183772, EP-482348, JP02-167838, JP01-242435, JP63-285137, JP09-156946. The Applicant observes that these still concern devices capable of carrying out a static measurement of the weight of the preform, and which therefore are subject to all of the errors in measurement mentioned above.
The Applicant has therefore specified a problem which is common to devices of the known type. Such a problem is linked to the fact that to measure the weight of the preform systems apt to carrying out a static measurement of the weight are used, such as for example load cells or sensors. In such systems, in particular, the weight is calculated by working out the vertical component of the forces acting upon the cells or sensors. The Applicant has verified that the measurement of the weight obtained through the use of load cells or sensors can be influences by various sources of errors, such as:                the drift of the signal caused by the heating of the load cells or sensors. Such a drift leads to high errors in measurement due to the non-uniformity of the thermal field which is realised on the cells. Such non-uniformity cannot always be eliminated through temperature compensation of the conventional type;        the action, on the cells or on the sensors, of loads which are different from the weight of the preform in formation. Indeed, the load cells are affected, besides by the weight of the preform, also by other static and dynamic types of force which it is not always possible to eliminate or quantify a priori. Static types of force typically comprise the weight of the preform anchoring systems, of the shafts and of the chucks associated with them, and of the systems for transmitting motion to the preform (typically belt or gear systems). Dynamic types of force, on the other hand typically comprise the dynamic actions of the systems for transmitting motion to the preforms. Another type of dynamic action is the inertia of the motion of the preform itself.        